Method and apparatus for detecting bark and for determining the degree of barking on wood and chips

ABSTRACT

The present invention relates to a method and to an apparatus for detecting the presence of bark and for determining the barking degree of wood or chips. Accordingly, the apparatus includes a light source (12) which directs a concentrated light beam (14) onto the measurement object (16), for instance a log which is moved continuously in a transport chute (20). The shape and/or size of the light image obtained on the log (16) at the point (18) on which the light beam (14) impinges on the log is detected with the aid of a camera (10), preferably a CCD-camera. The shape and/or size of the light image is mutually different for wood and bark. The detection values obtained by the CCD-camera (10) are evaluated in a processing means.

The present invention relates to a method and to apparatus forcontinuously indicating the presence of bark on such material as logsand chips, and/or for determining the extent to which such material hasbeen barked.

In the manufacture of wood-based paper pulp, it is extremely important,both with respect to product quality and production economy, to ensurethat bark has been removed from the wood with sufficient thoroughness.Depending on the final product desired, wood which is delivered to therefinery or to the digester may be allowed to have a maximum barkpercentage of 0.3-2%. The separation of bark from such material shouldnot, however, be undertaken too strenuously, since this will result inthe removal of wood together with the bark. This loss of good wood canhave a very noticeable effect on the production economy of the millconcerned. Thus, in the manufacture of paper pulp, it is extremelyimportant to debark the wood or the chips to an optimum degree. Thisoptimum barking degree also varies from product to product and betweenthe various manufacturing processes applied.

In order to achieve this optimum barking degree, it is not onlynecessary to adapt the barking process with respect to the product andthe process concerned, but also to the nature of the wood startingmaterial. Mutually different treatment times and treatment intensitiesare required when debarking different types wood, e.g. wood which hasbeen felled or logged at different times of the year, and also withrespect to the length of time the wood has been stored, the dryness ofthe wood, and whether or not the wood is frozen or thawed. The wood usedin a pulp mill is seldom of uniform quality, due to availability andvariations throughout the year, these variations often being unknown,and, consequently, it is often impossible to achieve an optimum degreeof debarking. It is therefore desirable to be able to determine thebarking result continuously and in a rational manner, so thatcorrections can be made in the bark separating process.

One significant drawback in this respect is that hitherto there has beenno objective method for determining the degree of barking of wood orchips rapidly. The only quick method used hitherto involves inspectingthe wood or chips visually, with subjective assessment of the extent towhich the wood or chips has been barked. This method, however, is notreliable and continuous inspection of the wood material is also veryexpensive and extremely tiring for the persons involved. The objectivemethods applied hitherto have been manual. One method is to separatebark from the wood or the chips and then dry and weigh the twoconstituents individually, such as to enable a measurement of the degreeof barking to be obtained. Alternatively, a measurement is taken of thetotal area of bark remaining on a log and this measurement compared withthe total mantle area of the log. Since these objective methods arehighly work intensive and expensive, they are only applied on singularoccasions, for instance when testing new installations or plants inorder to ensure that guarantees are fulfilled, or in the case of specialprocess studies.

SE-400 381 teaches a method and apparatus for the optical detection offaults in sawn or planed timber.

The apparatus includes, for instance, a light source and a lightdetector, each of which is located on a respective side of an opaquescreen having a thickness of about 2 mm. The light detector sendssignals to a comparator which produces an output signal when light thatfalls on the detector produces an electric signal which exceeds athreshhold value. A fault, for instance in the form of twigs,blue-stained wood, and certain types of decay, is indicated independence on the intensity of the light reflected. This apparatus canonly be used on sawn or planed timber, where the surface is so smoothand regular as to enable the opaque screen to be connected to thesurface of the timber, and where product requirements include relativelywell-defined requirements on the optical properties of the timbersurface with respect to colour, brightness, etc. The contour extensionsof logs or chips intended for pulp manufacture vary rapidly andextensively. Even though means could be provided which will enable thescreen to follow the surface of the wood material, the colour andbrightness of wood and bark would vary considerably, and consequentlythis known method would not be sufficiently precise. The method cannottherefore be used for detecting the extent to which logs or chips havebeen barked.

SE-404 964 teaches an apparatus for detecting radiation from an objectfor the purpose of establishing surface deviations or faults. This knownapparatus is intended for sorting timber, for instance green board,dried board and planks, and deviations in the form of bark residues aredetected through the intensity of the light reflected. For the samereasons as those given above, this known apparatus cannot be used fordetecting the extent to which logs or chips have been barked, since theorigin of the wood and the bark, their condition and the geometricalattitude of the surface, e.g. its inclination, etc. vary considerably.

These problems are solved by the inventive method and the inventiveapparatus, which have the characterizing features set forth in therespective characterizing clauses of claim 1 and claim 10 respectively.The inventive method and apparatus enable the degree of barking on woodor chips to be detected and determined continuously, therewith enablingthe separation of bark to be adjusted to an optimum barking degree onthe wood or the chips concerned.

One difficulty with detecting and determining the barking degree of woodor chips is that of distinguishing between bark and wood. Normally, itis expected that wood is light in colour and bark is dark. This is notalways the case, however, since wood can also be dark or coloured.Furthermore, wood can become dark or coloured as a result of storage,decay and like processes. A large part of the wood used in themanufacture of pulp has been stored over different lengths of time. Theinner bark of spruce, for instance, and also a number of other woodtypes, can also be light in colour, as can also the bark of birch.Consequently, it is not sufficient to utilize solely the brightness orcolour of the wood material when determining whether the object seen isbark or wood.

The method and the apparatus according to the present invention utilizedifferences in the optical properties of the bark and the wood, such asthe light scattering, light reflection and light absorption properties,for the purpose of determining the barking degree of wood or chips.Because of its long fibres, wood is a more orientated material thanbark. When concentrated light impinges on the surface of wood, the lightwill propagate within the wood material to a far greater extent in thelongitudinal direction of the fibres and the wood than in the transversedirection thereof. There is obtained an elongated light image. Thiseffect is generally greater in the case of wood than in the case ofbark. Thus, when light falls on wood and bark respectively, there isobtained a bark light image and a wood light image of mutually differentsize and shape.

The invention will now be described in more detail with reference to anexemplifying embodiment thereof and also with reference to theaccompanying drawings, in which

FIGS. 1a and 1b illustrate respectively the light images obtained withwood and with bark;

FIG. 2 illustrates the principle applied for adjusting the size of thedetected light image upon deviation from a nominal focusing distance;

FIG. 3 is a side view, partly in cross-section, of one embodiment ofapparatus constructed in accordance with the invention;

FIG. 4 illustrates the principle of an alternative embodiment of theinventive apparatus;

FIG. 5 illustrates the principle of a further alternative embodiment ofthe inventive apparatus;

FIG. 6 illustrates a method of distinguishing bark and wood-materialfrom background; and

FIG. 7 illustrates a further method of distinguishing bark andwood-material from background.

When practicing the inventive method and using the inventive apparatus,at least one concentrated light beam is directed onto material in theform of wood or chips, therewith to obtain a light image of thematerial. The light images thus obtained will have different shapes andsizes, depending on whether the light beam impinges on wood or bark.When the light beam, for instance a laser beam, impinges on a clean woodsurface, the light is spread in the wood fibres in a manner whichproduces an oval light image. See in this respect FIG. 1a. When thelaser beam, on the other hand, impinges on a bark surface, the light isspread to such a small extent that the light image obtained will havethe form of a spot of small diameter. See in this respect FIG. 1b. Thereference numerals 10 and 12 used in FIGS. 1a and 1b designaterespectively a detecting device and a laser. The light images shown inFIGS. 1a and 1b are illustrated in the same way as they are detected bythe detecting device 10.

FIG. 3 illustrates an embodiment of the inventive apparatus. Theapparatus includes a laser 12 which functions to direct a concentratedlight beam, in the form of a laser beam 14, onto a measurement object 16in the form of a log. The object 16 may also be a wood chip. At thepoint 18 at which the laser beam 14 impinges on the log 16, there isobtained a light image whose shape and size depends on whether the lightbeam 14 has impinged on wood or bark. (Compare FIGS. 1a and 1b). Theshape and size of the light image is detected by a detector device 10,in the form of a camera, preferably a CCD-camera having a field of view24. The use of a camera enables the light image detected by the camerato be determined in two dimensions, i.e. in size and shape. In the caseof simpler applications, it is sufficient to determine the area of thelight image by counting the number of picture elements of the camerawhich detects a light level which is greater than a threshhold value.Both the laser 12 and the camera 10 are housed in a respectiveprotective housing 22, so as to protect the laser and camera from theenvironment prevailing in wood-based pulp manufacturing mills. Each ofthe housings 22 is mounted on a respective rotatable plate (not shown),so that the angle between the laser 12 and the camera 10 can beadjusted. In turn, the rotatable plates can be displaced along amounting bar 26, so as to enable the distance between the laser 12 andthe camera 10 to be changed. The reference numeral 28 designates anelectronic system which includes process means for evaluating the signalreceived from the camera. The measurement object 16 (illustrated in theform of a log 16) is moved continuously past the measuring area on atransport chute 20 (shown in cross-section) to a cutter (where the logsare cut into chip form). The degree of barking is then determined by theprocessing device in accordance with the relationship: ##EQU1##

The surfacewise notation of the barking degree of the logs can beconverted to volume or a weightwise notation when the diameters of thelogs are known, or is measured, while taking into account the thicknessof the bark and the density of the wood and the bark.

FIG. 4 illustrates the principle of another embodiment of the inventiveapparatus, which comprises a plurality of lasers 12 which are arrangedon a line extending perpendicularly to the direction of movement of themeasurement object 16. In other respects, the apparatus comprises thesame components as those included in the apparatus illustrated in FIG.3. The lasers 12 need not necessarily be disposed along a line, but canbe arranged in any other suitable manner which will enable the materialto be illuminated in different positions transversely to the directionof movement.

FIG. 5 illustrates the principle of a further embodiment of theinventive apparatus, which comprises a laser 12 and a rotating prism 30which sweeps the laser beam 14 obtained from the laser 12 over themeasurement object 16. A camera 10 functions to detect thetwo-dimensional extension of the light image produced on the measurementobject 16 by the laser beam 14. The advantage with this principle isthat a large number of measurements can be made on each measurementobject and it is also possible to determine the diameter of said objectin accordance with known methods. Alternatively, there can be used alens arrangement such that the light beam, obtained for instance from alaser, will provide on the measurement object a linear light image,wherewith the width of the line can be determined on intermittentoccasions with the aid of a camera, in order to determine whether barkor wood is illuminated.

Because of the inability of the camera to focus at all ranges, it ispossible that the light images will appear to have different sizes.Compare FIG. 2. Whether or not this will cause a disturbance will dependon such features as the distance to the measurement object, the depth ofthe measurement area and the performance of the optics. In such case,the actual extensions of the light image can be determined prior todetermining the degree of barking. The actual or prevailing extensionsof the light image are determined by compensating for the deviation dfrom the focal distance of the camera, in accordance with therelationship: actual extension=measured extension-f(d). In the case ofstandard accuracy requirements, the following approximation can beapplied: f(d)=k·d².

In the embodiment illustrated in FIG. 3, the light source and detectorare arranged so that the deviation d, for instance, can be calculatedfrom the location of the detected light image in the camera viewingfield. Compare FIG. 2. The reference numeral 32 designates the measuringdepth of the apparatus. In this case, the focal distance of the cameralies in the proximity of the bottom end of the measuring depth. Themeasuring depth 32 is determined by the angle between the camera 10 andthe laser 12.

The manner in which the method and apparatus operate is also dependenton the ability to distinguish the bark and wood material from backgroundmaterial. One method of achieving this distinction is illustrated inFIG. 6, in which the background 34 is located at a distance from thetransport rollers 36 on which the measurement object 16 is transportedin the case of the illustrated embodiment. In the FIG. 6 embodiment, thelaser beam 14 falls at an angle such that a light point on thebackground 34 will lie externally of the area that can be observed bythe camera 10.

FIG. 7 illustrates an alternative method of distinguishing the bark andwood material from the background. In this case, the camera 10 and thelaser 12 are so positioned that only a small field-of-depth area 38,which is focused on the bark and wood material, is utilized during thedetection process.

Another method of solving the aforesaid problem is to permit thebackground to have optical properties different to those of bark andwood material.

We claim:
 1. A method for continuously establishing the presence of barkon a material such as logs and wood chips, and/or for determining thebarking degree of said material with the aid of at least oneconcentrated light beam directed onto the surface of the material andmoving relatively to said material, said method including the stepsof:detecting the size and/or shape of the light image obtained on saidsurface with the aid of the light beam, said size and/or shapeconstituting criteria relating to the constituents of the material,whereby an elongated light image is obtained when the light beamimpinges on a wood surface that is substantially free from barkdepending on the spreading of the light in the wood fibers and a lightimage in the form of a light spot is obtained when the light beamimpinges on a bark surface since the light is spread to a smaller extentin bark than on a wood surface that is substantially free from bark, andevaluating the detected values.
 2. A method according to claim 1,further comprising the step of employing linear illumination anddetecting the with of the light image on the material intermittently. 3.A method according to claim 1, further comprising the steps of sweepingthe light beam, e.g. in the form of a laser beam, over the material anddetecting the two-dimensional extension of the light image obtained onthe material.
 4. A method according to claim 4, further comprising thesteps of directing a plurality of the light beams onto the surface ofthe material, and detecting the two-dimensional extensions of the lightimages obtained on the surface of said material.
 5. A method accordingto claims 1-4, further comprising the steps of determining the actualextension of the light image by adjusting the extension of the detectedlight image in dependence on the distance between a detecting device,used to effect said detection, and the material.
 6. A method accordingto claim 5, further comprising the steps of determining the actualextension of the light image with the aid of the relationship:

    Actual Extension=Detcted Extension-f(d),

where, for instance, the following approximation can be used:

    f(d)=k·d2,

wherein d is a deviation from the focus distance of the camera.
 7. Amethod according to claim 1, further comprising the step of illuminatingthe material at an angle such as to make it impossible to observe alight image on the backround of the marerial.
 8. A method according toclaim 1, wherein said method being further characterized in that thematerial and the backround have essentially different opticalproperties.
 9. A method according to claim 1, further characterized byusing light in the wavelength range of 600-1300 nm.
 10. Apparatus forcontinuously establishing the presence of bark on a material such aslogs or wood chips, and/or determining the barking degree of thematerial, said apparatus comprising:means for producing a concentratedlight beam on the surface of the material; a detecting device whichfunctions to detect the size and/or the shape of a light image obtainedon said surface with the aid of the light beam, said shape and sizeconstituting criteria related to the constituents of the material,whereby an elongated light image is obtained when the light beamimpinges on a wood surface that is substantially free from barkdepending on the spreading of the light in the wood fibers and a lightimage with the form of a light spot is obtained when the light beamimpinges on a bark surface since the light is spread to a smaller extentin bark than on a wood surface that is substantially free from bark, andprocessing means which coact with the detecting device for evaluatingthe detected values.
 11. Apparatus according to claim 10, wherein themeans for producing the concentrated light beam is a laser. 12.Apparatus according to claim 11, wherein light in the wavelength rangeof 600-1300 nm is used.
 13. Apparatus according to claim 10, wherein thedetection device is a camera, e.g. a CCD-camera.
 14. Apparatus accordingto claim 10, further comprising a rotating prism which functions tosweep the concentrated light beam over the material, wherein theextension of the light image produced on the material is determined withthe aid of the detection device.
 15. Apparatus according to claim 10,further comprising a linear optic, which influences the concentratedlight beam in a manner such that the light image produced on thematerial will have the form of a line, wherein the width of said line isdetermined intermittently with the aid of the detection device. 16.Apparatus according to claim 10, characterized by one or more lightsources which produce a plurality of concentrated light images on thematerial (16), wherein the detection device functions to determine theextension of the light images.
 17. Apparatus according to claim 10,wherein the processing means functions to determine the actual extensionof the light image in dependence on the distance between the detectiondevice and the material in accordance with the relationship:

    Actual Extension=Detected Extension-f(d),

where, for instance, the following approximation can be used:

    f(d)=k·d2,

wherein d is the deviation from the focus distance of the camera.